1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, in which an image density of a toner image is adjusted based on detected image densities of patch images.
2. Description of the Related Art
This type of an image forming apparatus often sees a change in an image density due to the following factors: fatigue, degradation with age or the like of a photosensitive member and a toner; a change in a temperature, a humidity or the like around the apparatus; and other causes. Noting this, a number of techniques have been proposed which aim at stabilizing an image density through appropriate adjustment of a density control factor which influences an image density of a toner image such as an electrifying bias, a development bias, a light exposure dose, etc. For example, the invention described in the Japanese Patent Application Laid-Open Gazette No. 10-239924 requires to properly adjust an electrifying bias and a development bias in an effort to stabilize an image density. That is, according to this conventional technique, reference patch images are formed on a photosensitive member while changing an electrifying bias and/or a development bias and an image density of each reference patch is detected. An optimal electrifying bias and an optimal development bias are thereafter determined based on the detected image densities, and a density of a toner image is accordingly adjusted. For the convenience of description, in the following, the term a xe2x80x9cprocessing modexe2x80x9d will refer to a series of processing in which a plurality of patch images are formed, densities of the patch images are detected, and an optimal value of a density controlling factor, which is necessary to adjust an image density of a toner image to a target density, is determined based on the detected image densities.
The processing mode is executed at the following timing. Specifically, after turning on a main power source of the image forming apparatus, a density is adjusted upon arriving at a state where the apparatus is ready to form an image, which is when a fixing temperature reaches a predetermined temperature or immediately after that, for example. Where a timer is built within the image forming apparatus, the density adjustment is executed at regular intervals, e.g., for every two hours.
By the way, in a real image forming apparatus, a state of an engine part (image forming means) is largely different depending on an operation status of the apparatus. For instance, a change in a state of the engine part is relatively small while images are formed continuously, whereas it is relatively likely that a state of the engine part changes largely upon turning on of a power source.
Hence, execution of a processing mode tuned to the state of the engine part makes it possible to adjust a density efficiently at a high accuracy. For instance, while an optimal electrifying bias and an optimal development bias change due to fatigue, degradation with age or the like of a photosensitive member and a toner, the changes possess a continuity to a certain extent. Hence, when repeated density adjustment is desired, if a density is adjusted using a density controlling factor obtained from immediately previous density adjustment as a reference, the density adjustment is accurate. On the contrary, it is difficult to predict a state of the engine part upon power turn-on, and therefore, it is necessary to change the density controlling factor in a relatively wide range to determine an optimal value of the density controlling factor.
However, in conventional techniques, since only one type of a processing mode is available and the available processing mode is fixed, there is much to improve in terms of efficiency and accuracy.
The conventional technique described above requires to identify an electrifying bias/development bias characteristic before forming reference patch images, and to set an electrifying bias and a development bias for creation of reference patch images, such that the characteristic is satisfied. In order to stabilize an image density based on a calculated optimal electrifying bias and development bias, it is necessary to identify an electrifying bias/development bias characteristic of each image forming apparatus, which is troublesome.
Further, an electrifying bias/development bias characteristic does not always stay constant but may change with time. If the characteristic changes, it is difficult to accurately calculate an optimal electrifying bias or an optimal development bias. While appropriate updating of the electrifying bias/development bias characteristic solves this problem, the updating is bothersome and disadvantageous in terms of maintainability.
Meanwhile, other technique for stabilizing an image density is the invention described in Japanese Patent Application Laid-Open Gazette No. 9-50155. According to the described invention, a reference patch image, which is a patch image obtained by outputting groups of three-dot lines for every three dots, is formed on a photosensitive drum, and a sensor reads patch images thus created, whereby a line width is detected. A laser power is controlled based on the detected line width, a light exposure dose is accordingly adjusted so that a desired line width will be obtained, and an ideal line image is obtained.
However, a line image is basically a one-dot line which is drawn with one laser beam, and therefore, simply controlling a line width of a multi-dot line as in the conventional technique can not realize a precise adjustment of a line image.
The present invention aims at providing an image forming apparatus and an image forming method with which it is possible to determine an optimal value of a density controlling factor, which is needed to adjust an image density of a toner image to a target density, efficiently at a high accuracy.
Other object of the present invention is to provide an image forming apparatus and an image forming method with which it is possible to stabilize an image density of a line image.
In fulfillment of the foregoing object, an image forming apparatus and method are provided and are particularly well suited to density adjustment of a toner image based on image densities of a plurality of patch images.
According to the present invention, control means has a plurality of processing modes which are different from each other. Each of the plurality of processing modes is a mode in which a plurality of patch images are formed by the image forming means while changing a density controlling factor which influences an image density of an image and an optimal value of a density controlling factor, which is necessary to adjust an image density of an image to the target density, is determined based on the densities of the patch images. One of the processing modes is selected as a processing mode in accordance with an operation status of the apparatus. Hence, it is possible to select and execute the most appropriate processing mode in accordance with an operation status to thereby efficiently and highly accurately determine an optimal value of the density controlling factor.